Why is Python recursion so expensive and what can we do about it?

The issue is that Python has an internal limit on number of recursive function calls.

That limit is configurable as shown in Quentin Coumes’ answer. However, too deep a function chain will result in a stack overflow. This underlying limitation¹ applies to both C++ and Python. This limitation also applies to all function calls, not just recursive ones.

In general: You should not write² algorithms that have recursion depth growth with linear complexity or worse. Logarithmically growing recursion is typically fine. Tail-recursive functions are trivial to re-write as iterations. Other recursions may be converted to iteration using external data structures (usually, a dynamic stack).

A related rule of thumb is that you shouldn’t have large objects with automatic storage. This is C++-specific since Python doesn’t have the concept of automatic storage.


¹ The underlying limitation is the execution stack size. The default size differs between systems, and different function calls consume different amounts of memory, so the limit isn’t specified as a number of calls but in bytes instead. This too is configurable on some systems. I wouldn’t typically recommend touching that limit due to portability concerns.

² Exceptions to this rule of thumb are certain functional languages that guarantee tail-recursion elimination – such as Haskell – where that rule can be relaxed in case of recursions that are guaranteed to be eliminated. Python is not such a language, and the function in question isn’t tail-recursive. While C++ compilers can perform the elimination as an optimization, it isn’t guaranteed, and is typically not optimized in debug builds. Hence, the exception doesn’t generally apply to C++ either.

Disclaimer: The following is my hypothesis; I don’t actually know their rationale: The Python limit is probably a feature that detects potentially infinite recursions, preventing likely unsafe stack overflow crashes and substituting a more controlled RecursionError.

Why are recursive calls so much cheaper in C++?

C++ is a compiled language. Python is interpreted. (Nearly) everything is cheaper in C++, except the translation from source code to an executable program.

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